British astronomers may have detected light from a planet orbiting another star.

If so, this is the first time this has ever been done and a major achievement.

It builds on the extraordinary discoveries of recent years, which have revealed Jupiter-sized planets orbiting many of the stars nearest to our Sun.

Only last week, US astronomers announced they had seen a planet move across the face of a star and dim its light. But now scientists from the Department of Astronomy at St Andrews University, Scotland, claim to have detected the light reflected off a planet itself.

The team used the William Herschel telescope in the Canary Islands to observe the relatively nearby star Tau Boo. This star is slightly larger and brighter than our Sun and is 50 light years distant.

Bigger than Jupiter

Tau Boo was already known to have a world in orbit around it. In 1996, a team of American astronomers discovered a planet orbiting it every 3.3 days.

Measurements indicated that this planet had a mass of almost four times the mass of Jupiter, the largest planet in our Solar System. Spurred on by this discovery, the St Andrews team decided to look in detail at the spectrum of the star.

Hidden in the light from the star, they argued, must be a small amount of reflected light actually from the planet. Their problem was how to extract the planet's light from the starlight that was estimated to be tens of thousands of times brighter.

The clue was in knowing the orbit of the planet. With this knowledge, the astronomers were able to look at the spectrum of the star and extract that portion of the star's light that varied over the timescale of 3.3 days. This, they argued, must be the light from the planet.

'Oxygen discovered'

Although they do not have a picture of the planet, this is the first time that the light from another planet has been isolated. However, the astronomers are not yet ready to talk about their extraordinary discovery. When contacted by BBC News Online, they declined to comment or say if their research had yet been submitted to a science journal.

In analysing the light from the planet, the researchers are said to have obtained evidence that the planet's atmosphere is very hot and contains the elements magnesium, silicon and oxygen.

Detecting the light from a so-called exoplanet, a world circling another star, is a major astronomical achievement. The next step, obtaining an actual image of a planet, will probably have to wait for the next generation of space observatories that will come into operation in about 10 years' time.